Over 7,500 people showed up to Bernie Sanders' scheduled town hall forum in Portland, Maine on Monday night. After the crowds hit an estimated 2,000, the event was moved to the larger arena in town, which could hold 9,000 at full capacity.

It is the latest example of the independent socialist senator from Vermont turned 2016 Democratic presidential candidate attracting huge crowds on the stump in what was supposed to be a hopeless challenge to front-runner Hillary Clinton.

Sanders took to the stage twenty minutes late because event staffers struggled to find places for everyone to sit but the crowds were not upset with the wait. They cheered his name, as Sanders, energetic at age 73, smiled and looked around the arena, stating, "In case you didn't notice, this is a pretty big turnout."

Throughout all 55 minutes of his speech, the crowd was loud. Even as supporters interrupted his speech shouting expletives such as "You rock Bernie,you f---ing rock!", Sanders soaked it all in.

Sanders asked the question the media and the public alike have been musing over for weeks: How is he doing this well? The candidate decided to answer it himself.

"The answer, I think, is pretty obvious," Sanders said. "From Maine to California ... the American people understand that establishment politics and establishment economics are not working for the middle class."

During his speech, Sanders called income inequality "the great moral issue of our time." He said he wanted to create a movement in the tradition of the abolitionists, suffragettes, civil rights activists and union organizers to create substantial changes in politics and in America.

"If we want real change, it's not just electing someone — hopefully me," Sanders said to a laughing crowd. "No one in the White House will have the power to take on Wall Street alone, corporate America alone, the billionaire classes alone. The only way that change takes place is when we develop that strong grassroots movement, make that political revolution, stand together, and then we bring about change."

This wasn't Sanders' first big event. He attracted 10,000 people in Madison, Wis., over 5,000 in Denver, 3,000 in Minneapolis, and 2,500 in Iowa. So far Sanders is holding the largest campaign rallies of any 2016 hopeful.

Polls show Sanders gaining on Clinton in Iowa and within ten points of the former secretary of state in New Hampshire, the first two states of 2016 primary calendar.

"We are worried about him, sure," Clinton campaign communications director Jennifer Palmieri said Monday morning while on MSNBC. "He will be a serious force for the campaign, and I don't think that will diminish."